1. Field of the Invention
The present invention relates generally to electrochemical cells. More specifically, the present invention relates to enhanced membrane-electrode assemblies for use in such electrochemical systems and processes for making the enhanced membrane-electrode assemblies.
2. Description of the Related Art
U.S. Pat. No. 4,738,904, issued Apr. 19, 1988, and assigned to the present assignee, discloses a thermoelectrochemical system in which a continuous electrical current is generated from heat input below 250.degree. C. In this system, a hydrogen ion reacting cathode is immersed in a chosen Bronsted acid and a hydrogen ion reacting anode is immersed in a chosen Bronsted base. Reactants consumed at the electrodes during the electrochemical reactions are directly regenerated thermally below about 250.degree. C., and recycled to the electrodes to provide continuous operation of the system. The electrodes used in this type of thermoelectrochemical system are typically composed of platinum or other catalytic particles which are held together by a support network of microscopic hydrophobic fibers made from materials such as polytetrafluoroethylene (PTFE).
In operation of the above thermoelectrochemical system, hydrogen gas reacts with ammonia or an amine vapor in microscopic pores in the anode electrode to generate an electron and an ammonium ion or a protonated amine. The protonated amine ion is transported through a membrane in the cell to the cathode, where the protonated amine is transported into the microscopic pores of the cathode which are present between the platinum particles. Within these microscopic pores, a liquid acid and an electron react to produce hydrogen gas and an acid anion. The anion combines with the protonated amine ion to form a liquid salt.
The above-described thermoelectrochemical systems typically utilize "zero-gap" cells in which a thin electrocatalytic anode layer is bonded to one side of a thin membrane layer and a thin electrocatalytic cathode layer is bonded to the other side of the membrane. Nafion.RTM. is a cationic exchange membrane material available from E. I. DuPont de Nemours which is used as a common membrane material. Processes for bonding catalytic cathodes and anodes to Nafion.RTM. membranes are well-known in the art and typically involve thermal bonding.
Zero-gap cells which include Nafion.RTM.-type membranes are well-suited for their intended purpose. However, the cost of Nafion.RTM. membranes is relatively high and therefore precludes use of the these membranes for many commercial applications. Accordingly, it would be desirable to provide a zero-gap membrane-electrode assembly which is made using low-cost membrane material and provides electrochemical performance which is equal or superior to that obtained by cells utilizing Nafion.RTM. membranes.